The present invention relates to moving centrifugal gas separation and to mechanically assisted highly turbulent scrubbing of gaseous emission streams. No membranes or other dead-end filtration media nor any means for electrostatic separation are involved. This is a new method of carbon capture, suitable for high volume hot and dirty low concentration waste gas streams such as flue gas from coal-fired power plants.
The separative method of the present invention comprises differential radial advection of light and heavy fractions. Light fractions (including nitrogen and water vapor) are advected radially inward by back pressure and axial suction, while simultaneously heavy fractions (including carbon dioxide, NOx, SOx, and particulates) are advected radially outward by centrifugal impellers. Flue gas from coal fired power plants is the principal application, but other industrial waste gases, as well as natural gas, can be centrifugally separated and cleaned by the apparatus and method disclosed herein.
In U.S. Pat. No. 5,688,377 to McCutchen (1997) I disclosed a peripherally fed mechanically driven radial counterflow device for fluid mixture separation. Axial feed and application to flue gas for particulate and other scrubbing is a non-obvious improvement with the advantage that in a peripherally fed device such as McCutchen '377, the separated heavy fractions would mix with incoming feed. Therefore carbon capture would be impossible.
By the term gaseous emission stream is meant a gaseous fluid mixture such as flue gas from coal-fired utility and industrial boilers, exhaust from combustion of natural gas, diesel fuel, or oil, as well as emissions from municipal and medical waste incinerators, cement and lime kilns, metal smelters, petroleum refineries, glass furnaces, and sulfuric acid manufacturing facilities. A gaseous emission stream comprises non-condensible gases, such as carbon dioxide and nitrogen, as well as condensible gases and aerosols. The various constituents, whether gaseous, liquid, or solid, of a gaseous emission stream are fractions.
Wet scrubbing uses injected liquid to contact scrubbing targets in a gaseous emission stream. The injected liquid could be water, in the case of aerosol scrubbing, or an aqueous scrubbing slurry such as limestone and water in the case of SOx scrubbing. For NOx and SOx scrubbing, a chemical reaction occurs between a reagent in the injected liquid and the target fraction, producing harmless products. Carbon dioxide scrubbing by an injected aqueous amine solution is one means for carbon capture known to the art. Dry scrubbing of SOx injects limestone powder into flue gas. Wet scrubbing of fly ash agglomerates tiny particles by contact with water to produce a fly ash slurry.
The purpose of scrubbing is to separate a gaseous emission stream into a stream of concentrated unwanted fractions and a separate stream of desired fractions. For flue gas scrubbing, the unwanted fractions include fly ash and other aerosols, NOx, SOx, and carbon dioxide; desired fractions are nitrogen, water vapor, oxygen, and argon, all of which may be safely discharged to the atmosphere. For natural gas scrubbing, the desired fraction is methane and the unwanted fractions include carbon dioxide, hydrogen sulfide, water, particulates, mercury, and mercaptans.
Advection is transport responding to a pressure gradient or body force. The term radial denotes a direction outward or inward with respect to some point or line. For example, a centrifugal pump advects fluid radially outward from its axis of rotation.
Differential radial advection of a gaseous emission stream is advection of heavy fractions radially outward and simultaneous advection of light fractions radially inward. The reference line for said radial flow is the axis of rotation of said at least one centrifugal impeller. Different advecting forces act in opposite directions and on different constituents. Heavy fractions are advected radially outward by momentum transport from the centrifugal impeller. Light fractions are advected radially inward by axial suction and by back pressure, simultaneously.
At thermal equilibrium, the various fractions in a gas mixture have different average speeds. For example, at room temperature (300 K), carbon dioxide (CO2 molar mass 44 g/mol) has a root-mean-squared velocity, or average speed of its molecules, of 412 m/s, while nitrogen (N2 molar mass 28 g/mol) is 25% faster at 517 m/s, and sulfur dioxide (SO2 molar mass 64 g/mol) is 17% slower at 342 m/s. Water vapor (H2O, molar mass 18 g/mol) is the fastest fraction of all at 645 m/s. Halliday, et al., Fundamentals of Physics, 4th Ed., Table 21-1 p. 580 (1993).
The formula for radial acceleration, a (measured in units of g=9.81 m/s2), is a=v2/r where v is the velocity of the molecule and r is the radius of the curve of its path. When a mixture of said gases (at 300 K) is constrained to rotate in a 1 mm radius vortex, CO2 radial acceleration is 17.3 million g, N2 is 27.2 million g, and H2O is 42.4 million g. The light fractions (N2 and H2O) will be accelerated toward the vortex axis at higher radial acceleration than the heavy fractions (CO2 and SO2), although the centripetal force affecting all fractions is the same, 1.24×10−17 newtons. The radial acceleration of the light fractions, at a given vortex radius r, is much higher because of their their higher average speed.
Fine scale vortices will perform very high g separation of the fractions in the gas mixture because r in the formula is small. At higher temperatures than 300 K, vortex gas separation, due to the difference in radial acceleration of the light and heavy fractions, will be even stronger because the average speeds are higher for all fractions. The unsolved problem is how to force fine scale turbulence for vortex separation while collecting the separation effects. The present invention addresses that problem.
Coal Smoke.
Carbonaceous fuel combustion produces a gaseous emission stream called smoke. Each constituent of smoke is a fraction. Noxious fractions in smoke from coal-fired power plants include sulfur oxides (SOx, principally SO2 and SO3), nitrogen oxides (NOx, principally NO and NO2), aerosols (fly ash, mercury vapor, dust, trace metals), and carbon dioxide (CO2). There are also benign fractions (nitrogen gas, oxygen, and water vapor) which constitute approximately 85% of smoke by volume. Air used for combustion contains about 79% gaseous nitrogen (N2), which is inert, so flue gas is approximately 75% nitrogen. Water vapor (H2O) is created by combination of the hydrogen in the fuel with atmospheric oxygen during combustion. Much of the plume from flue gas stacks is water vapor forming a cloud as it contacts cool air.
The benign fractions in smoke have lower density (lower molar mass) than the noxious fractions. Therefore nitrogen gas, oxygen gas, and water vapor are referred to collectively as light fractions, and the noxious fractions (aerosols, NOx, SOx, and carbon dioxide) are referred to as heavy fractions. Carbon capture is the separation of carbon dioxide from the light fractions.
Coal is currently the dominant fuel in the power sector, accounting for 38% of electricity generated in 2000, with hydro power 17.5%, natural gas 17.3%, nuclear 16.8%, oil 9%, and non-hydro renewables 1.6%. Coal is projected to remain the dominant fuel for power generation in 2020 (about 36%), and natural gas power generation will become the second largest source, surpassing hydro. To combat the urgent problem of global warming, known post-combustion carbon capture methods (chemical sorption, membrane separation, and cryogenic distillation) would have to be scaled up to deal with the large streams of hot dirty dilute flue gas from coal fired and natural gas fired power plants. None has proved economically feasible so far.
Carbon Capture.
According to the IPCC Report on Carbon Capture (September 2005): “the power and industry sectors combined dominate current global CO2 emissions, accounting for about 60% of total CO2 emissions. Future projections indicate that the share of these sectoral emissions will decline to around 50% of global CO2 emissions by 2050 (IEA, 2002). The Co2 emissions in these sectors are generated by boilers and furnaces burning fossil fuels and are typically emitted from large exhaust stacks . . . . The largest amount of CO2 emitted from large stationary sources originates from fossil fuel combustion for power generation, with an average annual emission of 3.9 MtCO2 per source. Substantial amounts of CO2 arise in the oil and gas processing industries while cement production is the largest emitter from the industrial sector . . . . The ranges of the technical capture potential relative to total CO2 emissions are 9-12% (or 2.6-4.9 GtCO2) by 2020 and 21-45% (or 4.7-37.5 GtCO2) by 2050.”
Clearly there has long been a critical but unmet need for improved carbon capture from the gaseous emission streams of power plants. Two obstacles stand in the way of economical carbon capture: (1) the high volume percentage of nitrogen (˜75%), known as nitrogen ballast; and (2) the pollution of aerosols, NOx and SOx. Despite the efforts of many investigators having more than ordinary skill in the art of flue gas scrubbing or the art of centrifugal gas separation, no satisfactory answer to these two obstacles has been discovered. Any obvious steps using the teachings of McCutchen '377, which was published in 1997, or other prior art for the solution of such important problems, would have been evident long before now.
Three carbon capture processes are known to the art: cryogenic distillation, sorption, and membranes. The present invention introduces a fourth.
Cryogenic distillation captures carbon by liquefaction and separates out NOx and SOx by fractional distillation. Liquefaction is frustrated by nitrogen ballast, which compresses without liquefaction. The small partial pressure of NOx and SOx in flue gas (very much less than 1% by volume) and the small partial pressure of CO2 (10-15% by volume) are both due to the high nitrogen ballast (75%).
For sorption processes, nitrogen ballast makes carbon dioxide molecules like needles in a haystack, so an inordinate amount of wastewater is generated in contacting enough targets to achieve satisfactory collection efficiency. Fly ash, a fine silica dust, plugs absorbers used in sorbtion processes. NOx and SOx combine with water to become corrosive acids and heat stable salts, causing loss in absorption capacity and coating of reclaimer tube surfaces. Aqueous amine scrubbing of carbon dioxide from natural gas is much easier than the same process applied to flue gas polluted by fly ash, NOx, and SOx.
Membranes or other dead-end filtration media are impractical for large scale carbon capture from flue gas because of the presence of fly ash, which plugs the pores.
In view of these problems, a need exists for an alternative means for carbon capture, and for improved scrubbing upstream of known carbon capture processes. Here is the position of the U.S. Department of Energy: “The low pressure and dilute concentration dictate a high actual volume of gas to be treated. Trace impurities in the flue gas tend to reduce the effectiveness of the CO2 adsorbing processes. Compressing captured CO2 from atmospheric pressure to pipeline pressure (1,200-2,000 pounds per square inch (psi)) represents a large parasitic load.”
Both natural gas and coal-fired power plants emit voluminous streams of hot smoke, comprising 10-15% by volume carbon dioxide (CO2) at approximately atmospheric pressure. These are low-concentration/low-partial-pressure sources, which are the most difficult for carbon capture.
Nitrogen Extraction.
No means for extracting nitrogen from flue gas upstream of carbon capture are known. Nitrogen (N2) is a harmless gas which constitutes 75% of the volume of flue gas from coal-fired power plants. This is referred to as nitrogen ballast. Nitrogen might be safely discharged to the Earth's atmosphere, which is already 78% nitrogen. Other benign light fractions in flue gas are oxygen (02) (4%), water vapor (5%), and argon (1%). Altogether, approximately 85% of flue gas does not require any treatment at all, other than extraction. Extracting benign gases would increase concentration of carbon dioxide from only 10-15% in the gaseous emission stream to over 90%.
Centrifugal Gas Separation.
The molar mass of nitrogen gas (N2) is only 28 g/mol (grams per mole of gas); carbon dioxide (CO2) is 36% denser at 44 g/mol. Centrifugal gas separators which might exploit this 36% density difference are of two classes: mechanically driven and pressure driven.
Mechanically driven gas separators can exploit gas density differences as low as 1.5%, far beyond the performance required for flue gas separation. The ultracentrifuge is a very delicately balanced cylinder rotating at extreme speed and generating very high g force which radially stratifies gases by density within the cylinder. Such rotating cylinder centrifugal gas separators are dangerous because of their extremely high rotation speed.
Pressure driven devices include inertial collectors (also known as cyclones), and vortex tubes. Cyclones and vortex tubes are axial counterflow devices, wherein flow goes in opposite directions and the device is static.
Inertial collectors are used extensively to process gaseous emission streams to remove most aerosols. Cyclones have no moving parts. Tangential feed injected through the wall of a tank swirls downward along the wall in a first vortex, then swirls upward in a second vortex inside the first vortex. This is axial counterflow. Solids are centrifugated out against the wall of the tank and are collected at the bottom, and a cleaned gas exits the top with the second vortex. Cyclones, even cascaded, are ineffective even for the relatively easy job of separating out 2.5 micron fly ash. Nitrogen extraction by inertial collectors has not been reported and would appear to be impossible.
Another pressure driven centrifugal gas separation device without moving parts is the vortex tube. The net effect of a vortex tube is to separate a high pressure stream into two low pressure streams, one hotter and the other colder than the high pressure stream. Pressurized feed gas is tangentially injected into one end of a static tube having both ends open. The feed gas spirals in a first vortex to the opposite end, where there is a conical central flow impedance. Hot gas exits the tube around the flow impedance. A recirculation flow rebounds from the flow impedance in a second vortex inside the first vortex and exits the feed end cooler than the feed gas. Residence time in the vortex tube is on the order of milliseconds.
Application of the vortex tube has also been made to the separation of liquefied air into nitrogen and oxygen, to removing condensible vapors from natural gas, and to improving sorbent mixing for carbon dioxide scrubbing. The presence of fly ash in flue gas, the high energy requirement for pressurizing the feed, and the poor separation efficiency for gas/gas separation (gas fractionation) would appear to make the vortex tube unsuitable for extracting nitrogen from flue gas.
Fly Ash and Other Aerosols.
Aerosols comprise fly ash, soot, condensible vapors, mist, and dust. These fractions are airborne because they are very small. Scrubbing of aerosols agglomerates these fractions so they can be separated more easily downstream. However, approximately 6% by mass of particle emissions from pulverized bituminous and sub-bituminous coal combustion is in the form of aerosols too small to separate by known processes or devices.
Fly ash is fine inorganic (principally silicon dioxide) particulate matter formed during coal combustion. The most troublesome fly ash is in the form of minute silica dust less than 2.5 millionths of a meter (micron) in diameter (PM-2.5). Collected fly ash is valuable as a concrete additive and as a material for making durable and impervious bricks which require no firing. Fly ash can therefore be seen as both a problem and an unexploited resource.
Soot, another particulate emission, is uncombusted fuel, which is usually not a problem in power plants where combustion is complete. Combustion is frequently not complete, and in gaseous emission streams from ships and vehicles, soot is a serious problem.
Other aerosols include vapors, mist, dust, and trace metals. Mercury and VOCs (volatile organic compounds) are condensible vapors which are regulated emissions because of their known harmful effect. Mist is tiny liquid droplets, including sulfuric acid droplets, water droplets, and droplets from condensed condensible vapors. Dust is airborne fragments of inorganic material. Trace metals in flue gas include uranium, arsenic, lead, cobalt, chromium, and thorium.
Dry electrostatic precipitators (ESPs) are the principal means used for collecting aerosols from coal-fired flue gas. Other industries using ESPs for emission control are cement (dust), pulp and paper (salt cake and lime dust), petrochemicals (sulfuric acid mist), and steel (dust and fumes). A cathode in the flow path of a gaseous emission stream imparts a negative charge to the entrained particles. A positively charged collector plate (anode) downstream in the flow path attracts the negative charges. Charged aerosols adhere to the collector plate and agglomerate in a coating. The coating is dislodged by rapping into a hopper.
ESPs, when working properly and with the right fuel, may have an overall collection efficiency as high as 99.2%. Where ESPs fail is in collecting fly ash under 2.5 microns and other fine particulates. The size limit for effective aerosol collection in ESPs is approximately 10 microns.
Even lower collection efficiency for fine particulates is found in the performance of inertial collectors, such as cyclones. Estimated overall control efficiency for a cascade of multiple cyclones is 94%, but fine particulates mostly escape collection. Cyclones are often used as a precollector upstream of an ESP, fabric filter, or wet scrubber so that these devices can be specified for lower particle loadings to reduce capital and/or operating costs.
Wet Scrubbers for NOx, SOx, and Aerosols.
The 94% overall collection efficiency for particulates in prior art wet scrubbers is inferior to the maximum collection efficiency of ESPs. Mechanically aided wet scrubbers known to the art spray liquid onto centrifugal fan blades as waste gas flows through the fan. The advantage of mechanical assistance is less water usage and a smaller footprint. Collection takes place in the spray and in the film that forms on the fan blades.
Wet particulate scrubbers have the disadvantage of trading an air pollution problem for a water pollution problem.
Venturi scrubbers, the most turbulent of wet scrubbers, have the highest collection efficiency. Venturis are pressure driven devices which jet a combined stream of waste gas and liquid through a nozzle into a tank. However, venturis have the disadvantage that turbulence quickly dissipates into pressure. The time during which turbulent mixing occurs is short.
For NOx and SOx, the injected scrubbing liquid is an aqueous solution comprising sorbents (lime or limestone for SOx, ammonia for NOx). The sorbents react with the target gases to produce harmless products.
Removal efficiency for prior art wet SOx scrubbing ranges from 50-98%. As with all prior art wet scrubbers, the price paid for high removal efficiency is a large volume of dilute wastewater requiring storage and treatment and a consequent waste of space and resources. Dry SOx scrubbers have a lower removal efficiency, <80%, but do not create wastewater.
A known carbon capture process used in processing natural gas is wet scrubbing with aqueous amine solution. NOx, SOx, and aerosols in flue gas complicate the application of this known technology to high volume hot and dirty exhaust gas streams.
Low concentration of SOx and NOx (less than 1% by volume) in a high volume of nitrogen and other benign fractions means that a large amount of liquid must be injected in order to contact enough of the scarce scrubbing targets to achieve satisfactory removal efficiency. Low concentration also means that condensation is more difficult, due to low partial pressure.